Sign binary packages for Mac OS X

From QTBUG-8314:

The Mac binary packages should probably be signed using the MacOS-specific signing method. For a little extra overhead, this would give users confidence that the Qt libraries come from us.

See following link for more information:

http://developer.apple.com/mac/library/documentation/Security/Conceptual/CodeSigningGuide/RequirementLang/RequirementLang.html#//apple_ref/doc/uid/TP40005929-CH5-SW1

Note from Daniel Birns:
Note that Qt would have to supply nametool patched release versions, since changing the symbol links in the deployed frameworks would invalidate the signature.